1. Field of the Invention
The invention pertains to a a para-aromatic polyamide filament yarn.
2. Description of the Prior Art
Para-aromatic polyamide filament yarn is known from EP 609 946 (van der Pol), which discloses p-aramid yarn where the filaments may have a linear density of 0.8 to 1.3 dtex, the example given being 1.1 dtex. Van der Pol teaches that yarns characterized by a lower filament linear density than that of standard yarn have several advantages. For instance, this known yarn has excellent properties for use as a reinforcing fiber in rubber articles which can be subjected to mechanical load. However, there is still room for substantial improvement. For instance, EP 609 946 fails to provide an aromatic polyamide yarn which can be put to advantageous use in a wide range of applications. Thus the search is on for p-aramid yarn having a high internal shear modulus for use, int. al., as antiballistic yarn, but also for use as reinforcing yarn in optical cables, and for fabrics to be utilized without a matrix or with matrices other than rubber. This so-called g-value constitutes a proper standard for a number of properties which are relevant to the yarn all at the same time, viz. drawing modulus, torque modulus, and axial compression strength. Yarn such as described by Van der Pol has a comparatively low g-value of about 2.2 GPa. It should be noted that while this value can be increased. Other publications also disclose yarns with filaments having a lower linear density than standard yarn.
For instance, JP-Hei-6-2216 (Teijin) describes PPTA yarns having a filament linear density of 0.17 to 0.75 denier (converted: 0.19 to 0.83 dtex). It is stated that these yarns especially possess favorable abrasion resistance. However, the described yarns are less suited to be used in actual practice on account of their low linear density thereof. Among other things, this low linear density means that the yarn cannot be usefully employed as such, but only in an assembled form. This needless assembling of yarns is an economically unattractive additional step and, moreover, involves the risk that the mechanical properties of the yarn finally assembled will have decreased proportionally. The described process, which typically produces bundles of 133 and maximally 300 filaments having a yarn linear density of at the most 100 denier (110 dtex), does not readily allow thicker yarns to be produced. In the case of a thicker bundle use will have to be made of a smaller air gap if the pitch/air gap ratio required according to Teijin is to be maintained. Coagulating a thick bundle also is more difficult when using this process. Furthermore, Teijin has the drawback that the obtained yarn has an extremely low crystallite length (L002 value). Besides, Teijin when spinning p-aramid from solution does so with a very low polymer content in the spinning solution and a very high acid concentration in the spinning bath, as a result of which the described yarns inherently lack the optimum properties of the PPTA. In addition, the high acid concentration in the bath leads to sticking of filaments, particularly in the case of thicker bundles. Lower acid concentrations are not possible in the Teijin process, because low concentrations lead to high yarn cutting. Besides the aforementioned low L002 value, yarns made as specified by Teijin have an objectionably high para-crystallinity (indicating a highly disturbed crystal structure). Another drawback to the yarns described by Teijin is their low density.
Chimitsjeskie Volokna, No.2, pp 17-19, March-April 1993 (Kiya-Oglu et al.) describes the manufacturing of PPTA yarns using an air gap-wet spinning process with a far greater degree of drawing in the air gap than is customary. It is stated that this can be done only when the temperature in the air gap is greatly reduced. For instance, at -35.degree. C. there was 65.times. drawing. Although the publication of Kiya-Oglu et al. has no bearing on yarns having a low filament linear density, it is self-evident that said linear density will be reduced by the drawing. Thus the yarn drawn up to 65.times. is composed of filaments having a linear density of 0.26 dtex. The yarns described here do not have the yarn linear density desired in actual practice either (the number of filaments is restricted to 200). Moreover, the low temperature in the air gap required according to this publication is highly unattractive from a practical and economical point of view. Nor does this process make it possible to produce thicker bundles. For, when the number of filaments is higher, it proves impossible to create a homogeneous situation as regards the temperature and the drawing characteristics in the air gap, which has a detrimental effect on the product properties. Also, blowing a larger number of filaments involves practical problems.
The use of yarns having a lower than standard linear density for ballistic applications is known in itself. For instance, EP 241 681 (Droste) teaches a bullet resistant vest in which use is made of a fabric of aramid yarn with a filament linear density of less than 1.5 dtex, the example given being 1.12 dtex. In actual practice, yarn with a filament linear density of 0.93 dtex has also become popular.